Elderly female subjects from the Aging Process Study on variable fixed intakes of ascorbic acid (vitamin C) had significantly higher (30%) plasma ascorbic acid concentrations compared to males. In reviewing the possible reasons for this difference, i.e., absorption, metabolism, volume of distribution, and renal clearance, we hypothesize that this might be due to a difference in renal clearances of ascorbic acid in the two sexes. We propose to determine renal thresholds and tubular reabsorptive maximums.